Here’s a little update:
I tried adding the classes in tinymce.functions.php, but then suddenly tinyMCE stopped working altogether!
Probably wrongly escaped double quotes, but I don’t have time to fiddle around with that now...
FYI:
I replaced
$tinymceInit .= (!empty($advimage_styles) ? " advimage_styles : \"".$advimage_styles."\",\n" : "");
with:
$tinymceInit .= " advimage_styles : \"Copy Image Right=copy-image-right\"\",\n";
approx. at line 180.
What DID work, however, was to change assets/plugins/tinymce212/jscripts/tiny_mce/plugins/advimage/image.htm
<select id="classlist" name="classlist">
<!--
original line:
<option value="" selected>{$lang_not_set}</option>
-->
<option value="">{$lang_not_set}</option>
<option value="copy-image-right">Copy Image Right</option>
</select>
Ugly? Maybe. But it works